Method for the production of fibrous sheet materials

ABSTRACT

In the production of fibrous sheet materials by an air-laying process adjusting the moisture content of the fibrous layer at a value of at least 6% by weight and embossing said fibrous layer at a temperature of above 100 degrees C.

June 13, 1972 T. B- RASMUSSEN 3,669,778

METHOD FOR THE PRODUCTION OF FIBROUS SHEET MATERIALS Filed Feb 5, 1970 3 Sheets-Sheet 1 A8 Z: Mo/ls tune conzent l1. 6 8.

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\Bfm) y, tfmbassi/y fem o. 250C 300 5 20o c g 150 C g4 2 w /Z0 OC y T ,5 m mmrg) content 7 INVENTOR fizl BY June 13, 1972 T. B. RASMUSSEN 3,669,778

METHOD FOR THE PRODUCTION OF FIBROUS SHEET MATERIALS Filed Feb. 5, 1970 3 Sheets-Sheet 5 I: 'laboss/ry tem 250 t 2 i H 76 c 0 6,0 (76) Binder INVENTOR ATTORNEY US. Cl. 156-62.2

United States Patent 3,669,778 METHOD FOR THE PRODUCTION OF FIBROUS SHEET MATERIALS Torben Borup Rasmussen, Abyhoj, Denmark, assignor to Karl Kristian Kobs Kroyer, Aarhus-Viby, Denmark Filed Feb. 3, 1970, Ser. No. 8,180 Claims priority, application Great Britain, Feb. 4, 1969, 5,943/ 69 Int. Cl. B29j 5/04 4 Claims ABSTRACT OF THE DISCLOSURE In the production of fibrous sheet materials by an airlaying process adjusting the moisture content of the fibrous layer at a value of at least 6% by weight and embossing said fibrous layer at a temperature of above 100 C.

BACKGROUND OF THE INVENTION This invention relates to a method for the production of fibrous sheet materials comprising the steps of moistening, embossing and treating a fibrous layer with a binder, which predominantly consists of organic natural fibres and which has been prepared by a dry process. The invention also relates to the fibrous sheet materials made by said method.

Fibrous materials prepared by such a method tend to delaminate because of the difliculties encountered when attempting to cause the binder to penetrate into said fibrous layer. It has been attempted to reduce this tendency of delamination by impregnating the fibrous material with great amounts of binder. However, this technique presents the drawback that the fibrous materials become rigid and that the costs of production are considerably increased due to the fact that the binder is a relatively expensive component of said materials.

The object of the invention is to obtain a product having a reduced tendency of delamination without using excessive amounts of binder.

SUMMARY OF THE INVENTION This object is obtained, according to the invention, by a method which is characterized in adjusting the moisture content of the fibrous layer at a value of at least 6% by weight and heat embossing the fibrous layer at a temperature of above 100 C. before it is sized.

During said wet and heat embossment of the fibrous material of the above mentioned type the organic natural fibres used are deformed and the contact area in the points of intersection are significantly increased. This results in the obtaining of a capillary effect which causes the absorption capacity of the fibrous material to be significantly increased. Furthermore, the deformation of the fibres in the contact areas increases the strength of the fibrous material.

Due to the increased absorption capacity the penetration of the binder is improved within such areas, i.e. the contact areas, within which the effect of the binder is optimum compared to a fibrous product which is embossed in cold and dry condition.

This is evidenced by an increase of the breaking length of the fibrous material. For a cold embossed fibrous material the breaking length increases gradually with increasing contents of binder up to a content of about 50% whereafter the breaking length decreases. In a wet and heat embossed fibrous material the breaking length increases very rapidly up to a binder content of about 30% and then starts decreasing. However, the breaking length at a binder content of about 30% is considerably higher than that of a cold embossed fibrous material having the optimum binder content of about 50% Thus, the same strength can be obtained by the method according to the invention by using far less binder than in the prior art methods. As mentioned above the total moisture content of the fibrous layer should be at least 6%. It has been found by experiments that with increasing moisture content the breaking length is markedly increased at a total moisture content of about 6% and that said increase of the breaking length continues when the moisture content is further increased. The increase of the breaking length continues up to a total moisture content of about 15% whereafter a further increase of the moisture content does not further increase the breaking length.

The increasing moisture content requires the use of increased amounts of energy to dry the product and consequently it is normally undesirable to increase the total moisturecontent of the fibrous layer to a value above 15%.

As mentioned above the embossing temperature should be above 100 C. Thus, it has been found that when embossing at temperatures below 100 C. the embossing does not cause the breaking length to be increased. However, at 100 C. the breaking length is suddenly increased and said breaking length increases with increasing temperatures. Due to the fact that cellulosic fibres are miscoloured at temperatures of about 260 C. it is normally undesirable to operate at such high temperatures. A particularly preferred embossing temperature for normal cellulosic fibres is about 250 C. provided the embossing speed is suitably adjusted to said temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will now be described with reference to the following example:

(A) Embossment of a fibrous sheet In order to illustrate the effect of the embossing temperature, speed, pressure and the moisture content of the the product on the breaking length the tests mentioned below were carried out.

The breaking length is calculated based on the following equation:

wherein B=breaking length expressed in metres P=the sum of breaking loads expressed in kg.

n =the number of tests g=the weight of the sheet expressed in grams per square metre b=the width of the sample expressed in cm.

In these tests an embossing roller set consisting of a steel embossing roller and a smooth rubber roller was used. The diameter of the steel embossing roller was 240 mm. and the length was 400 mm. It was electrically heated by heating elements having a total effect of 16 kilowatt allowing the temperature of the roller to be adjusted at desired values of up to 350 C. The diameter of the rubber roller was 100 mm. and its hardness was Shore.

The embossing pattern used corresponds to that of velvet having a spacing between the embossed areas of 1.4 mm. and with embossed areas having a width of 0.3

All tests were carried out with samples having a length of 18 cm. and a width of 2.5 cm. If moistened the samples had been treated by means of a pressurized air atomizer.

Based on the data obtained curves were prepared show- 3 ing the relationship between the breaking length and the variable factors.

FIG. 1 shows the relationship between the breaking length, the temperature and the moisture content of the fibrous sheet, the embossing speed, the embossing pressure and the weight of the sheet being constant. The embossing speed used was 75 metres per minute, the embossing pressure 17 kg. per cm. and the weight of the sheet was 70 grams per metre It appears from FIG. 1 that the increase of the breaking length by using temperatures up to 100 C. isinsignificant irrespective of the moisture content of said sheet. By using a moisture content of 6.6% the breaking length suddenly increases at 100 C. and a further increase is obtained at temperatures up to about 260 C. The curve showing the breaking length of a sheet having a moisture content of 6.6% illustrates that the curve flattens at a temperature of about 160 C. This is due to the fact that the major part of the water has been evaporated at temperatures of between 100 and 150 C. and that the amount of water present at temperatures of about 160 C. are so insignificant that no further deformation of the fibres takes place.

FIG. 2 shows the breaking length as a function of the embossing speed and the moisture content, the embossing temperature and the embossing pressure being constant. It appears from FIG. 2 that at low embossing speeds a maximum breaking length is obtained when embossing fibrous products having a relatively high moisture content.

FIG. 3 illustrates the relationship between the breaking length, the embossing pressure and the embossing ternperature, the moisture content being constant. It appears from FIG. 3 that the strength is not increased significantly when operating at higher roller pressure than 17 kg. per

FIG. 4 illustrates the breaking length as a function of the moisture content and the embossing temperature. It appears from said figure that irrespective of the embossing temperature the breaking length is not increased significantly at a moisture content of up to about 6%. At a total moisture content of between 6 and 10% the breaking length is significantly increased by using embossing temperatures of between 120 and 250 C. When the moisture content is above approximately 12% the increase of the strength is insignificant. However, the product becomes rigid and relatively high amounts of energy should be used to dry the product. As mentioned above the increase of the strength used by hot embossing moist fibrous sheets is due to a permanent deformation. When embossing a fibrous sheet by means of cold rollers the fibres are merely compressed and due to their flexibility they tend to resume their original shape after the embossing.

(B) Sizing or the treating with a binder of embossed fibrous sheets Due to the permanent deformation which primarily takes places in the points of contact between the fibres and which increases the contact areas a significant capillary elfect Within said areas is obtained. This is partly due to the fact that the air present between adjacent fibres and preventing the penetration of the binder is squeezed out.

The strength of the embossed and binder treated fibrous sheets is shown in FIG. 5 which illustrates the breaking length as a function of the amount of binder expressed in percent of the weight of the sheet material. In the tests carried out the moisture content before the em bossing was 10.7% and an embossing pressure of 17 kg. per cm. was used to emboss a sheet material having a weight without binder of grams per metre 2 at an embossing speed of metres per minute. The binder used was an acryl latex in a concentration of 25% (solid content). The binder was applied by means of a pressurized air atomizer.

Curve 1 in FIG. 5 shows the results obtained by testing a fibrous sheet material which had been embossed at a temperature of 250 C. and the curve 2 shows the data obtained under similar conditions except that the fibrous product had been embossed at a temperture of 15 C.

FIG. 5 shows that with increasing contents of binder of up to about 30% the breaking length increases. At that point the breaking length decreases. The tensile strength also increases with increasing contents of binder but the breaking length decreases because the weight of the sheet material is increased at a relatively higher rate than the tensile strength.

When using a fibrous sheet material which has been embossed by means of cold rollers the breaking length increases with increasing contents of binder of up to about 50% at which point the breaking length starts decreasing. When comparing the above mentioned two curves it is evident that the maximum breaking length for the hot embossed product is significantly higher than that of the cold embossed product and that the maximum breaking length for the hot embossed product is obtained at a binder content which is significantly lower than that of the cold embossed product.

It. should also be pointed out that the hot embossed product containing no binder has a considerably higher breaking length than that of the cold embossed product.

I claim:

1. A method for the production of fibrous sheet materials comprising the steps of passing a stream of gas containing suspended organic natural fibres through a gas permeable forming surface to form a fibrous layer thereon and embossing and treating said fibrous layer with a binder, characterized in adjusting the moisture content of said fibrous layer at a value of at least 6% by Weight and embossing said fibrous layer at a temperature of above C. before it is treated with said binder.

2. A method according to claim 1, characterized in adjusting the moisture content of the fibrous layer at a value of between 6% and 15% by weight.

3. A method according to claim 1, characterized in embossing the fibrous layer by means of rollers at a temperature of about 250 C. and at peripheral speed of about 75 111. per minute. I

4. A method according to claim 3, characterized in embossing the fibrous layer at a pressure of about 17 kg. per cm.

References Cited UNITED STATES PATENTS 2,760,881 8/ 1956 Toulmin, J r. 15 6209 2,803,188 8/1957 Duvall 101--32 3,235,913 2/ 1966 Schuller 15662.2 X 3,467,541 9/1969 Aronsson et al. 162-207 X BENJAMIN A. BORCHELT, Primary Examiner H. J. TUDOR, Assistant Examiner US. Cl. X.R. 15 6209 

